In Near-Reef Waters of French Polynesia (Society and Tuamotu Islands)

BULLETIN OF MARINE SCIENCE, 48(1): 15D-158, 1991

HIGH CONCENTRATIONS OF TUNA LARVAE (PISCES: SCOMBRIDAE) IN NEAR-REEF WATERS OF FRENCH POLYNESIA (SOCIETY AND TUAMOTU ISLANDS)

J. M. Leis, T. Trnski, M. Harmelin- Vivien, J.-P. Renon, V Dufour, M, K. El Moudni and R. Galzin

ABSTRACT Plankton samples at 5 m and 10m were taken weekly 100-200 m from coral reefs off each of three islands in French Polynesia (Moorea, Rangiroa, Takapoto) for 4 weeks in January- February 1989. Tuna larvae were among the most abundant fish larvae captured, and were present in 95.8% of the 72 samples taken. Thunnus spp. (T. a/bacares and T. a/a/unga), Katsuwonus pe/amis, and Auxis spp. were commonly taken in moderate to large numbers. Euthynnus affinis was uncommon. The larvae were generally small with average size 2.6-3.5 mm depending on taxon and island, indicating spawning nearby. Tuna larvae were present in high concentrations. Average concentrations were well above those reported for the oceanic

central Pacific, and the maximum concentration (223.7' 500 m-3) is among the highest re- corded anywhere during the day. Thunnus spp. were more concentrated at 5 m than at 10 m. Other taxa did not differ in concentration between 5 and 10m. Differences in concentration were found among the islands, but these patterns of difference varied among species. All islands had similar temporal patterns for Katsuwonus pe/amis larvae, but patterns for Thunnus spp. larvae differed among islands. The implications of such high concentrations of tuna larvae so close to reefs are discussed.

Tunas of the family Scombridae (Katsuwonus and Thunnus) are generally considered archetypal oceanic fishes (Joseph et aI., 1980; Sund et aI., 1981). They are highly migratory, efficient predators of the epipelagic zone, and are assumed to complete their life cycles in the open ocean. Based on this assumption, most studies of tuna larvae have taken place in oceanic waters, far from land. Such studies in the Pacific have resulted in very low average concentrations of tuna larvae (Table 1), surprisingly low, in fact, given the apparent abundance of adults. One possible explanation for this apparent shortage of tuna larvae is that the larvae are concentrated near islands or reefs and not in the open ocean. A possible advantage of such a nearshore distribution is higher productivity and plankton biomass near islands which could result in greater food availability for larval tuna. A variety of physical processes (reviewed by Hamner and Wolanski, 1989) result in increased nutrient availability and retention of plankton near islands; one manifestation of this has been termed the island mass effect (Doty and Oguri, 1956). Further, concentrations of reef fish larvae-potential food for older tuna larvae (Davis et aI., 1990)-are often high near reefs (Leis and Goldman, 1984; Leis, 1986), thus providing another possible advantage for near-reef distributions. Alternatively, oceanic islands and associated reefs may act as aggregation devices for spawning adult tunas (T.L.O. Davis, pers. comm.), but provide no particular advantages for larvae. In French Polynesia, for example, adults of at least some tunas are more abundant within 40 km from islands than further from shore (Petit and Kulbicki, 1983), and the best catch rates of adult scombrids are found close to atolls or over shoals (Villiers and Meyer, 1983). Studies which have looked for offshore gradients of tuna larvae abundance around islands have not found them (Nakamura and Matsumoto, 1966; Higgins, 1970), but the sampling may have been at inappropriate scales. For example, Nakamura and Matsumoto (1966) considered 'inshore' to be less than 75 nautical 150 LEIS ET AL.: POLYNESIAN NEAR-REEF TUNA LARVAE 151

Table 1. Mean concentrations (number' 500 m-3) and abundances (number' 10 m-3) of tuna larvae reported in the oceanic Central Pacific. Mean concentrations in the present study were 22.7-35.6, depending on location. See text regarding abundance estimates for the present study

Concentration Location (abundance) Gear and tow type Season Reference Central 2.0 I-m net, oblique & year-round Strasburg, 1960 Pacific (2.4 to 60 m) surface (8.0 to 200 m) Marquesas 1.8 I-m net, oblique Oct-June Nakamura and Ma- (2.5 to 140 m) tsumoto, 1966 French 0.35* 2-m net, oblique & Oct-March Nishikawa et aI., Polynesia (-*) surface 1985

• Note that Davis et al. (L989) report the Japanese 2~m net may underestimate concentrations of tuna larvae by a factor of 1l.8 to 39.6. Based on unspecified mixture of surface and oblique lOWS, so abundance cannot be calculated. miles from land, and Higgins' inshore station was 7 km from shore. There are, however, indications that larvae oftuna and billfish-another archetypal oceanic group- may be highly concentrated less than 1 km from coral reefs (Miller, 1979; Leis et aI., 1987). Our purpose here is to report very high concentrations of tuna larvae in waters less than 200 m from reefs in the Society and Tuamotu Islands in French Polynesia. We also provide limited information on vertical, horizontal and temporal distribution. We cannot, at present, explain these high concentrations, but our results combined with those of Miller (1979) suggest the possibility that tuna larvae may regularly occur in very high concentrations very near islands and reefs.

MATERIALS AND METHODS

We sampled at three islands of French Polynesia in the central South Pacific: Moorea (17°30'S, 149°50'W), a high island in the Society Islands, and two atolls in the Tuamotu Islands, Rangiroa (150$, I48°W) and Takapoto (14°40'S, 145°10'W). Rangiroa is about 350 km NE of Moorea, and Takapoto is about 290 km ENE of Rangiroa. These are typical oceanic islands: they lack a continental shelf and the bottom quickly drops away to oceanic depths (see Delesalle et aI., 1985 for additional information). Samples were taken 100-200 m from the reef crest off the western side of each reef. This is normally the lee side. Water depth varied among and within sampling sites from 15 m to 100-200 m. The bottom, where it could be determined, was coral reef. Identical methods were used at all three sites. An 85-cm diameter ring net (two point bridle), fitted with a flowmeter, and of 0.5-mm mesh was towed from a small (ca. 7 m) outboard-powered boat. Tows were done at I m·sec-I, and we attempted to sample 500 m3 during each tow. Actual volume sampled ranged from 282-616 m3• Mean (and standard deviation) of the volumes sampled at each island were: Moorea 443.6 (82.9), Rangiroa 475.5 (57.9), and Takapoto 454 (71.2). Horizontal, strat- ified tows using the methodology of Leis (1986) were made at two depths: 5 m and 10 m. Samples were taken weekly at all islands over the period 22 January to 15 February 1989. Three replicate sets of depth-paired tows were taken at each island between 0800 and 1230 on one day each week, for a total of 24 samples from each island. Nets were washed down with seawater, and the samples fixed in 5-10% formalin in seawater. Samples were sorted with the aid of a dissecting microscope at 5-10 x without subsampling. Tuna larvae were identified after Matsumoto et a1. (1972), and Nishikawa (1988). Standard length oflarvae was measured to the nearest 0.05 mm using an eyepiece graticule. Larvae per sample were converted to larvae· 500 m-3 (i.e., concentration), because 500 m3 was the nominal volume sampled. Multiple comparisons were made using the Friedman test (Conover, 1971) and the Least Significant Range procedure (a Student-Newman-Keuls procedure based on ranks: Zar, 1974; Leis, 1982). A P < 0.05 was considered sufficient to reject a null hypothesis of no difference.

RESULTS Scombrids were among the most abundant larvae in our samples. Wahoo larvae, Acanthocybium solandri, were present in small numbers, but will not be considered 152 BULLETIN OF MARINE SCIENCE, VOL. 48, NO. I, 1991

Table 2. Summary of catches of tuna larvae near reefs in French Polynesia (concentrations in number' 500 m-3)

Taxon Moorea Rangiroa Takapolo Auxis sp(p). Positive tows 8.3% 75.0% 0% Concentration Range 0-1.8 0-20.9 0 Median (mean) 0(0.11) 4.4 (6.7) Size range 2.3-3.1 mm 1.8-5.0 mm EUlhynnus affinis Positive tows 8.3% 16.7% 0% Concentration Range 0-2.8 0-3.5 0 Median (mean) 0(0.13) 0(0.27) Size range 2.5-2.8 mm 2.3-3.5 mm Kalsuwonus pe/amis Positive tows 87.5% 79.2% 41.7% Concentration Range 0-76.4 0-38.2 0-40.6 Median (mean) 5.5 (12.5) 4.3 (9.9) 0(5.0) Size range 2.0-5.2 mm 2.1-5.2 mm 2.0-7.3 mm Thunnus spp. Positive tows 79.2% 95.8% 91.7% Concentration (5 m) Range 0-57.0 1.0-114.2 0-223.7 Median (mean) 4.1 (10.0) 13.4 (29.1) 10.2 (32.8) Concentration (10 m) Range 0-10.1 0-34.7 1.0-58.1 Median (mean) 3.6 (4.1) 6.9 (11.8) 6.6 (15.3) Size Range 1.9-4.9 mm 1.8-5.8 mm 2.1-7.0 mm Total Tunas Positive tows 91.7% 100% 95.8% Concentration Range 0-100.1 3.6-114.2 0-223.7 Median (mean) 10.8 (22.7) 35.6 (37.5) 22.5 (29.7)

further. Tuna larvae of five species of four genera were captured. Skipjack, Kat- suwonus pelamis, larvae were captured in large numbers. Thunnus larvae were abundant, and were represented by T. alalunga (albacore) and T. albacares (yel- lowfin). These two species generally cannot be distinguished at sizes below 4.5 mm (Matsumoto et al., 1972), and as most of our specimens were smaller than this, we refer to them as Thunnus spp. Both species were represented among larger larvae: 14 T. alalunga and 1 T. albacares. All the Thunnus larvae identifiable to species were taken at Takapoto. We captured smaller numbers of frigate tuna, Auxis spp., the species of which cannot be distinguished as larvae (Nishikawa et al., 1985). A few kawakawa, Euthynnus affinis, were captured. The taxa in rank order of abundance and occurrence were Thunnus, Katsuwonus, Auxis and Eu- thynnus. The tuna larvae we captured were small: only 28 of the 2,252 larvae were flexion or postHexion stage. Mean size ranged from 2.6-3.5 mm depending on taxon and island (Table 2). Many of the smaller larvae still retained some yolk. Tuna larvae were present in 91.7-100% of the samples (depending on island, Table 2). Thunnus spp. was the most commonly-occurring taxon, and was present in 79.2-95.8% of the samples. Concentrations were high (Table 2), with medians LEIS ET AL.: POLYNESIAN NEAR-REEF TUNA LARVAE 153

Table 3. Differences in concentration of tuna larvae between paired tows at 5 m and 10 m. Values arc concentration at 5 m minus that at 10 m in numbers' 500 m-J• N is number of pairs of samples in which the taxon occurred. P from Wilcoxon signed rank test (Conover 1971), 95% confidence interval from Dixon and Massey (1957)

Taxon Median (95% el) N p

Auxis sp(p). -3.0 (-11.1 to +3.6) 10 >0.20 Katsuwonus pelamis -0.9 (-1.9 to +1.4) 28 >0.20 Thunnus spp. + 1.1 (-0.1 to + 10.8) 35 <0.02

of 10.8-35.6 per 500 m3• At Takapoto, concentrations of tuna larvae (all taxa combined) reached 223.7 per 500 m3. Variation among replicate samples was high at all three islands. The variance to mean ratio for replicate samples averaged 17.4 (range 0.008-105.6). Differences in concentration of tuna larvae between 5 and 10m were not detected for Auxis or Katsuwonus (Table 3). Thunnus larvae were found in higher concentrations at 5 m than at 10 m, but the difference was small (Table 3). Where differences in occurrence or concentration of tuna larvae were found among the three islands, Takapoto had the lowest values. Auxis larvae were absent at Takapoto, and were present in only 8% of the samples at Moorea, but they were present in 75% of the tows at Rangiroa. Concentrations of Katsuwonus larvae were significantly lower at Takapoto than at the other two islands which were not significantly different from each other (Friedman/LSR test). Concentrations of Thunnus larvae were tested separately for 5 and 10m because of the difference in vertical distribution found in this taxon. Analysis of both the 5 m and 10 m data gave similar results: Moorea had lower concentrations than the other islands, but the differences were not significant (Friedman/LSR test). We used Fischer's method to combine the probabilities of these two tests (Sokal and Rohlf, 1981), but the result remained non-significant (P > 0.2). Temporal patterns on concentration over the 4-week sampling period (Fig. 1) varied widely among the three abundant taxa (all were tested by Friedman/LSR). Auxis larvae occurred in enough samples for analysis only at Rangiroa where the pattern was week 2 > 1 = 3 > 4. Katsuwonus had the same pattern at all three islands: 3 > 1 = 2 = 4. Thunnus had a different pattern at each island: at Moorea, 3 = 4 = I > 2; at Rangiroa, 4 = 2 = I > 3; at Takapoto, 4 > 2 = 3 = 1. The Thunnus temporal patterns were the same if data from the two depths were tested separately.

DISCUSSION Our sampling at the three stations considered here was intended to serve as an outside "control" for more intensive sampling of reef-fish larvae within the la- goons at the three islands. We had not set out to sample scombrids, and were surprised to find such high numbers of tuna larvae consistently present over a 4-week period. The mean concentrations ofscombrid larvae we encountered were well above those reported for oceanic waters in the central Pacific (Table 1). We encountered concentrations oftuna larvae up to 223.7,500 m-3-higher than any reported elsewhere during the day except the extremely high concentrations (to 10,945' 500 m-3 in patches 5-15 km in diameter) of Thunnus maccoyii found in the NE Indian Ocean by Davis et al. (in press). The next highest day time catch- 220·500 m-3-was reported by Miller (1979) also inshore near a coral reef. It is worth noting that the 0.5-mm mesh used by us probably undersamples yolk-sac 154 BULLETIN OF MARINE SCIENCE, VOL. 48, NO. I. 1991

• MOOREA • RANGIROA ..•. TAKAPOTO

...... ME Karsl/\\'ol/l/S pelal11is o o lJ')

c .Q 100 .•...• (\j .•...••... C Q) Thl/I/I/l/S Spp 5m () C o ()

Thl/I/I/IIS spp 10m

o 1 2 3 4 Week Figure 1. Temporal patterns to concentration of tuna larvae (Auxis, Katsuwonus and Thunnus) in near-reef waters of three islands in French Polynesia. Plotted values are means, Auxis larvae were abundant enough for analysis only at Rangiroa. Thunnus concentrations were shown separately only 5 and 10 m because concentrations differed between these depths (see text), For other tunas, data from 5 and 10m were combined. tuna larvae, so actual concentrations are probably higher than we report, but most studies of tuna larvae use such mesh, so reported catches are comparable. A rigorous comparison of abundance (larvae· 10 m) between near-reef and oceanic areas is not possible because we almost certainly did not sample the entire vertical extent of tuna larvae (Davis et al., 1990). Further, some of the oceanic LEIS ET AL.: POLYNESIAN NEAR-REEF TUNA LARVAE 155

data cannot be expressed as abundance (Table 1). However, if we assume that tuna larvae near reefs occur almostly exclusively in the upper 10m (i.e., where we sampled), abundance can be estimated. The resulting abundance values will almost certainly be too low because the assumption that tuna larvae do not occur below 10m is almost certainly false. However, estimates of near-reef average abundance of 4.5-7.5'10 m-2 result (depending on island, with a maximum of 44.7,10 m-2), and these are as high or higher than average abundance estimates based on the upper 60-200 m offshore (Table 1). So, even conservative estimates of abundance nearshore are at least as high as abundance reported offshore. High concentrations of tuna larvae within 200 m of coral reefs have now been reported in Hawaii and French Polynesia. We encountered high concentrations of Auxis, Thunnus and Katsuwonus. In Miller's (1979) Hawaiian work, Thunnus and Auxis were the abundant taxa, but Miller sampled at the surface during the day where Katsuwonus is rarely encountered (Davis et aI., 1990). Recent work around Oahu along a transect between I and 15 nautical miles offshore detected highest concentrations of Thunnus, but not other tuna larvae, at I mile offshore (G. W. Boehlert, pers. comm.). This pattern of distribution was more distinct off the lee side than off the windward side of Oahu. In addition, in near-reef areas of the Atlantic, Richards (1975) reported that larval tuna concentrations on the Florida side of the Florida Straits were similar to those found near reefs by us, but that few tuna larvae were found on the Bahamas side of the Straits. If such high concentrations of tuna larvae near coral reefs are a common occurrence, there are several important implications. First, surveys of tuna larvae which examine offshore waters only may be missing a high proportion of the larvae in spite of the very much greater area of offshore waters. Second, growth and mortality rates may differ inshore and offshore. If they do, they may be more favorable inshore (Theilacker, 1986), therefore making larvae from inshore waters disproportionately important. Third, high concentrations inshore facilitate studies of such characteristics as vertical distribution and temporal fluctuations which may be obscured offshore by low numbers, a point made previously by Miller (1979). Fourth, surveys of tuna larvae inshore using small vessels could be carried out more cheaply than oceanic surveys, and by using multiple small vessels as in this study, simultaneous sampling can be carried out. It is possible that the high catches of tuna larvae inshore merely reflect the superior temporal and spatial scale of sampling inshore as compared to offshore in the Indo-Pacific (T.L.O. Davis, pers. comm.). However, the high proportion of inshore samples which contain tuna larvae during the spawning season (this study, and Miller, 1979) indicate this is not the case unless patchiness of larval distributions is fundamentally different in the nearshore areas. Davis et al. (in press a) in the NE Indian Ocean showed that extremely high concentrations of at least Thunnus maccoyii may occur offshore, but they are very limited spatially, and probably very limited temporally due to restricted spawning season and short duration of the larval period. This makes detection of any oceanic high concentrations very difficult. The offshore sampling effort for tuna larvae (Strasburg, 1960; Nishikawa et aI., 1985) has been very thinly spread considering the vast areas involved, particularly if small patch sizes reported by Davis et al. (in press) are typical. This could explain why offshore high concentrations of other species have not generally been found (Table I) assuming they exist. The high concentrations we encountered enabled detection of small differences in vertical distribution at fine scales, although it is possible vertical distributions inshore could differ from those offshore. Most previous studies of vertical distri- 156 BULLETIN OF MARINE SCIENCE, VOL. 48, NO. I, 1991 bution of tuna larvae (summarized by Davis et aI., 1990) have either considered coarse scales (e.g., above vs. below the thermocline), or have compared surface with oblique tows. These studies have concluded that skipjack larvae avoid the surface during the day. We did not sample the surface, but found high concentrations of skipjack near the surface (at 5 and 10 m) during the day. Thunnus larvae are considered to be surface oriented during the day (Davis et aI., 1990). This is consistent with our finding higher concentrations at 5 m than at 10 m. However, concentrations were still high at 10 m. We did not sample the surface or deeper than 10 m, so little more can be said regarding vertical distribution, but tuna larvae certainly are found at depths greater than 10 m. The temporal patterns evident in our samples imply an area-wide synchrony (on a scale of weeks) in spawning intensity by skipjack. The differences in temporal pattern among locations for the other taxa could reflect more localized spawning by the other species, or variations in physical factors such as currents. The spatial patterns evident in our samples suggest that larval distribution is not random over the area, but that some locations may be better or worse than others for tuna larvae (at least over 4 weeks). We sampled at only one "outside" station at each island, so we cannot say if we have detected a difference among islands or a difference among locations having nothing to do with islands. Miller (1979) found differences in concentration of tuna larvae both among locations and among islands in Hawaii. The Florida side of the Florida Straits has far higher concentrations of tuna larvae than the Bahamas side (Richards, 1975). So, consistent differences among locations are not unprecedented. We do not pretend to provide a definitive test, but our results are not consistent with the hypothesis that wind-induced upwelling on the lee side of islands (Miller, 1979) is the cause of nearshore high concentrations of tuna larvae. This is based on two observations. Miller's (1979) hypothesis requires that tuna larvae occur in highest concentrations away from the surface. This is contrary to what is known of the day time vertical distribution of Thunnus (Davis et aI., 1990), the most abundant tuna taxon in both Miller's samples and our own. Second, Miller's hypothesis requires that the wind pushes surface water away from the lee side of the island. This is then replaced by upwelled water containing high concentrations of tuna larvae. On six of the 12 occasions that we sampled, there was no wind, and therefore, no wind-induced upwelling. This included all islands on week four when the highest concentrations of Thunnus larvae were encounterd at Takapoto and Rangiroa. We do not know if tuna complete their larval phase in near-reef waters such as those studied here. If they do, Takapoto is the most likely location because we found the largest larvae there (Table 2). Many of the larvae we encountered still had yolk, which shows that spawning takes place relatively near the reefs (within a few days drift, at most), although perhaps not as close as 200 m. We captured few large larvae. This could be due either to absence nearshore or to gear avoidance. However, the upper tail of the size-frequency distribution we encountered is very similar to that found by Davis et aI. (1989) using a net very similar to ours offshore in the Indian Ocean. Because tuna are generally considered to complete the larval stage offshore, but both studies captured few large larvae, avoidance is implicated. If so, completion of the larval phase near reefs cannot be ruled out. We cannot say how far from the reefs the high concentrations we encountered extend. We did not sample offshore, so it is not impossible that the concentrations we encountered are typical of the open ocean in the area. However, the low LEIS ET AL.: POLYNESIAN NEAR·REEF TUNA LARVAE 157 concentrations reported for tuna larvae in the oceanic Central Pacific (Table 1) make this very doubtful, and we preliminarily conclude that an on-offshore gra- dient in tuna larvae concentration exists. This conclusion is supported for Thunnus by recent work in Hawaii (G. W. Boehlert, pers. comm.). Published information on on/offshore gradients provides little assistance. Miller (1979) could not detect a difference in concentration between < I km and <2 km from shore in Hawaii. No difference in concentration over a Hawaiian transect running from 0.2 to 3 km offshore was detected by Leis (1982). Clearly, given the important implications if near-reef waters prove to be important nursery grounds for tuna larvae, further investigation is warranted. Our study raises a number of questions about tuna larvae which need to be addressed, particularly the distance off the reefs to which high concentrations extend, the degree to which survival inshore differs from that offshore, and the extent to which these concentrations of larvae inshore represent local populations. However, it still remains to be demonstrated that high inshore concentrations represent high abundances, that inshore values are higher than offshore values obtained at the same time, that inshore samples are in any way representative of events offshore, or that larvae found inshore eventually recruit to adult populations. The present evidence is largely circumstantial, and the sampling was not designed to address these questions, or even to study tuna larvae. Further work on the question of near-reef distribution of tuna larvae will be needed to answer these questions, but given the potential importance ofthe subject, the effort seems warranted. A final point concerns management of tuna fisheries. Obviously, further research on tuna larvae near reefs is needed to determine if near-reef waters are essential or important in the life history of tunas. However, if they are, anthropogenic impact on near-reef waters will be of concern to tuna fishery management. Of perhaps equal importance is that the countries controlling these near-reef(usually near-shore) waters will be able to make powerful claims of ownership of tuna stocks if near-reef waters are vital to the life history of the fishes. This could greatly alter our view of tunas as "fish without a country" (Joseph et al., 1980).

ACKNOWLEDGMENTS

This research was supported under the Australian-French Joint Coral Reef Research Program (DITEC 88/5692; PICS-CNRS no.77). Additional funding was provided by the Australian Museum Trust. P. Cabral made available EVAAM facilities on Takapoto and Rangiroa. I. Moeroa, 1. Algret and A. Lefevre assisted in the field. T. L. O. Davis and B. Salvat constructively criticized the manuscript, and the former provided copies of several of his important papers prior to publication. Our thanks to all. This is a contribution to the Antenne EPHE/Museum Research Station, Moorea.

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DATEACCEPTED: July 20, 1990.

ADDRESSES: (f.M.L. and T.T.) Division of Vertebrate Zoology, The Australian Museum. P.O. Box A285, Sydney South, N.S. W. 2000, Australia; (M.H.- v., f.-P.R., V.D., M.K.E.M. and R.G.) Antenne de Recherches Museum - E.P.H.E., Centre de /'environement de Moorea, B.P. 1013, Papetoai, Moorea, Polynesie Franr;aise; in addition, (M.H.- V.) Centre d'Oceanologie de Marseille, URA CNRS No 41, Station Marine d'Endoume, rue de la Batterie-des-/ions, 13007 Marseille, France; PRESENTADDRESS: (f.-P.R.) Laboratoire d'Ecologie Animale-Zoologie, UFR Faculte des Sciences, BP 6759, Universite d'Orleans, 45046 Orleans Cedex, France; (V.D., M.K.E.M. and R.G.) Ecole Pratique des Hautes Etudes, Universite de Perpignan, 66025 Perpignan Cedex, France.